1. Field of the Invention
The present invention relates to apparatus and methods for electron beam imaging.
2. Description of the Background Art
A conventional electron beam apparatus commonly includes, among other modules, an emission module, a beam current selection module, and a resolution optimization module. The conventional emission module typically uses either a thermal field emission (TFE) emitter with a relatively larger source energy spread or a cold field emission (CFE) emitter with a relatively smaller source energy spread. The conventional beam current selection module typically includes a single focusing lens configured together with fixed beam-limiting and column apertures. The resolution optimization module typically includes condenser and objective electron lenses. The focus of the condenser lens and the focus of the objective lens are commonly varied to find an optimal convergent angle at which the electron beam spot size at the target wafer is minimized.